Although experts said the viruses discovered so far were relatively benign, the news represents a blow to Apple, whose software has avoided the relentless virus attacks that have plagued Microsoft’s Windows operating system.

News of the security threat emerged last week after Macintosh users reported that an image file purported to contain screen shots of a forthcoming OS X update actually contained a computer worm. Since then, at least two other OS X security threats have emerged, including several variants of a worm that propagates through a vulnerability in the popular BlueTooth hands-free software suite.

Andrew Welch, a software developer who helped pick apart the first OS X virus after it was discovered on Thursday, said he was tempted to label the discovery a “non-event”.

“[The virus is] poorly written, can’t spread beyond your local network, is unlikely to infect anything on most machines, and needs user interaction to do anything at all,” he wrote on a bulletin board located on his software company’s website.

F-Secure, a Finnish anti-virus company, said that three variants of a Macintosh computer worm discovered on Tuesday could be classified as “proof of concept” viruses because they contained internal controls that allowed them to work only in a controlled environment, and not in the real world.

However, it warned that hackers could develop similar viruses that lacked such controls and could spread “in the wild”.

The Apple platform’s relative security compared with Microsoft’s Windows operating system has long been a point of pride among Macintosh fans.

But some industry watchers have maintained that Apple’s relative safety is due mostly to the fact that Microsoft, whose software powers more than 90 per cent of the world’s computers, makes a bigger, more tempting target for enterprising hackers.

That may be beginning to change as Apple basks in the glow of its popular iPod personal music player. The iPod’s so-called “halo effect” was thought to be partly responsible for a big increase in converts to the Macintosh platform last year.

Shares in Apple rose 0.2 per cent to $69.08.

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